Understanding the molecular basis of interferon-alpha (IFNalpha) action is an important goal when one considers IFN's therapeutic potential in cancer, viral hepatitis, and multiple sclerosis, as well as its role as a model for understanding cytokine signal transduction. IFNalpha elicits its biological actions by regulating gene expression through the tyrosine phosphorylation and activation of members of the STAT (signal transducers and activators of transcription) protein family. The applicant found that STAT3, a transcription factor for acute phase response genes, is a critical element in IFN signaling and induction of IFN's biological actions. In addition, it was also found that IFN promotes cell survival by activating NF-kB (nuclear factor-kB) through a serine kinase-dependent pathway involving PI-3K (phosphatidylinositol-3' kinase) and Akt, as well as STAT3. Based on these findings, the general hypothesis to be tested is that the IFNalpha receptor integrates signaling pathways involving STAT3, PI-3K and NF-kB. In Specific Aim 1, the role of STAT3 as a transcription factor and an adapter protein for PI-3K will be defined. The proposed studies will determine which specific amino acid residues in STAT3 undergo IFN-dependent phosphorylation, the relationship of these phosphorylation events to the biologic actions of IFN, and which IFN-responsive genes are STAT3- regulated. In Specific Aim 2, the role of NF-kB in IFNa action will be defined. The proposed studies will define the relationship between PI-3K/Akt-mediated phosphorylation events and the anti-apoptotic action of IFN, the roles of TRAFs (TNF receptor-associated factors) and NIK (NF-kB-inducing kinase) in IFNinduced NF-kB activation, the role of NF-kB in gene induction by IFN, and the role of the IkB kinase complex in IFN promoted NF-kB activation and cell survival. Despite advances made on the IFNalpha signaling pathway, the mechanisms that underlie the induction of the different biological actions of IFNalpha remain poorly understood. This proposal focuses on characterizing the molecular basis of signaling pathways as they relate to IFN action on cell proliferation and survival.